In theory that approach could somewhat work, but in practice you run into problems with arranging the MOSFETs so the body diodes never conduct, and with their gate drive. Generally a MOSFET requires approximately double its max. gate threshold voltage to turn on properly, and the gate-source voltage needs to be under half the min. gate threshold voltage to turn off properly, and I think that with both source voltages varying rail to rail, and the gate drive shared by both MOSFETs you risk neither MOSFET turning on, or possibly even both turning on!
Consider using a SPDT analog switch IC controlled by the comparator.
Alternatively, have you considered that it may be preferable to multiply the two control voltages so Lightburn's 0%-100% becomes proportionately 0%-80% or whatever upper limit you set on the machine control panel?